Vacuum tube with filamentary cathode



Feb. 22, 1949. v EEN 2,462,441

VACUUM TUBE WITH FILAMENTARY CATHQDE Filed Feb. 12, 1947 INVENTOR. JOHNA. VIGTOREEN Patented Feb. 22, 1949 NT OFFICE VAGUUM TUBE WITHFILAMENTARY CATHODE John A. Victoreen, Cleveland, Ohio ApplicationFebruary 12, 1947, Serial No. 728,109

3' Claims. 1.

This invention relates to improvements in thermionic tubes and moreparticularly to an improved filamentary cathodefor. avacuum tube and.the method of. making, the same. See my copendi'ng application. Number642,446.

Heretofore, many problems have arisen inv connection with themanufacture of vacuum tubes, not the. least of which were those arisingin conjunction. with the construction and operation of the filament.Among, other things, the size of the filament was one of the controllingfactors in: the. construction. In large tubes this did not involve toomuch of a-problem, but in small tubes, the limitations as to the size:of the filament has a definite bearing in its construction. This willbebetter appreciated when itis. understood. that in a. vacuum tube it-isdesirable to heat the filament. w th some conventional voltage source,and that the temperature of the filament and its resistance is usually afunction of its length and circular mil area as well as the specificresistance of the material.

In the smaller type tubes, the filaments are very oftenv heated by 1.4or 1.5 volt voltage sources, such as a dry; wet, or so-called Ma1lorycell. In. order. for a filament to beheated to the desired emittingtemperature by a 1.4 or 1.5 volt cell, it was necessary to: provide oneof a certain length. The length was also determined by the specificresistance of the current necessary to heat the filament to an emittingtemperature varied with the diameter, it was. desirable to draw the wireto a diameter as small as possible. In order to use. prior materials,the filament had to be longer which made it necessary to fold thefilament inorder to get it into a small space. The folded filament,although taking. up less longitudinal space, took a greater transversespace, thus placing limitations on the spacing of. the tube elements.Furthermore, the. folded or the long filament had a tendency to vibratewhen shocked which resulted in microphonic operation, as well asinstability thereof.. The filament, being long and of small diameter,had less strength and. there was, therefore, a tendency for it to sag,causing short circuits- It. also. had many other disadvantages,including that of complicating the manner of its support. Small diameterfilaments also had a greater tendency toward hot spots due tounequalities in diameter which occurred during. the drawing process.

;Another disadvantage of. the previous filaments was thevariation ofthe. resistance of the niaterlal upon. heating. Normally, all priorfilament, and since the 2 known filaments had a high temperaturecoefiicient of resistance which resulted in a low resistance when cold,and which resistance greatly increased upon heating. This variation inthe temperature coefficient of resistance in nickeL.

for instance, is of the order of 400% from hot to cold. This increase inresistance or change in resistance after the filament is heated is. veryundesirable because it is the direct. cause of circuit instabilities.The fact that the voltage source which is used to heat the filamentvaried during its life, being maximum when new and dropping off afteruse, caused. very apparent changes in the tube performance and resultantchanges in circuit performance due to the inability of the current inthe filament to follow proportionate changes in voltage. Suchinstabilities were such that in certain instances. the utility of thetube was greatly limited.

Furthermore, in these prior filaments, and, tungsten in particular, whenfirst heated, had a low resistance; but this resistance increased as.the heating was prolonged. Thus, the. resistance did not keep pace withthe heating, causing. the

, filament to glow brightly at first and then gradually decrease inbrightness to its normal color. This flashing effect had a tendency todecrease the life and. stability of the filament. Furthermore, changesin ambient temperature were. sufficient to cause resistance changesresulting in changes in the emission current. These changes wereespecially undesirable when. the tubes were used in conjunction with adirect current amplifier, because in a direct current. amplifier theabsolute values of emission deter-- mines what happens in the circuit,as distinguished from an alternating current amplifier where onlyincremental changes are of interest. This effect may be utilized incertain places, such as a bolometer where a high temperature co.-efi'icient is necessary to its operation, but has noplace in a vacuumtube, and more particularly, is extremely undesirable when the tubes areused for instrumentat on as in nuclear physics, where exceptional staticemission stability and circuitv resistance stability is desired.

By my present invention I have discovered animproved filament materialwhich has a. high specific resistance as compared with previously knownfilament. This enables a much shorter filament to be used thanheretofore. By the use of a short filament, longitudinal and transversespace in the tube are economized. This enables a very small tube to. bemade. Itenables the filament. to be. heated by conventional voltagesources, such as 1.5 volt cell, instead of a fraction thereofgthusgreatly enhancing its use. lhe manner of supporting the filament issimplified, allowing a more sturdy support, causing the fila ment to bemore rigid and eliminating all the disadvantages of the long flexiblefilament. The filament is freely suspended at its ends Without havingcontact with any other part of the assembly. Thus there is no thermalcontact with other parts of the tube which would cause the plate currentto assume diiferent values due to mechanical disturbances.

Furthermore, the filament of my discovery has a low temperaturecoefilcient of resistance being of the order of 5 percent from hot tocold. Still another advantage of the filament of my invention is that inmy filament, which has low thermal conductivity, a greater efiiciency ofthe filament in operation is realized, because the filament heats moreevenly throughout its length and heats to the desired temperature closerto its support and there is less heat lost throughout the support. This,as distinguished from the conventional tungsten or nickel filament whichhas its highest temperature at the places most re moved from its supportand which is quite cool at its pointsof support due to its high thermalconductivity.

To better illustrate certain aspects of the invention, reference will behad to the accompanying drawing wherein;

Fig. 1 is a full sized elevational view of a tube embodying the filamentof my invention;

Fig. 2 is an enlarged vertical medial section thereof; v

Fig. 3 is a graph depicting the operation of the tube of my invention ascompared with a commercial hearing aid tube.

The filament of the present invention is par ticularly useful in theso-called miniature tubes, such as is shown in Fig. 1. In the tube, theoverall length is approximately 1/ inches, and the spaceinside theenvelope which houses the tube element is '7 inch long. The diameter ofthe tube is approximately /32 inch, and'the inside diameter isapproximately .275 inch. The actual useful portion ofthe filament is /32inch long while the diameter is .0004 inch and is provided with acoating of oxides which is .0002 inch thick,'bringing the diameter ofthe complete filament with its oxide coating to .0008 inch.

The construction of such a tube is shown in detail in the greatlyenlarged sectional view in Fig. 2. Here the glass envelope which housesthe elements is shown at l0. A pair of ceramic members are provided at Hspaced from the top and bottom of the tube and held in position bysupports I2 which extend through the press I 3 at the base. One of thesupports is adapted to support a getter l4 and the anode I5 is supportedbetween the two ceramic members. The filament l6 has its lower endsecured in the lead l8 preferably by insertion in the hollow end of thelead which is then pressed and welded. The upper end of the filament issupported by a spring wire of U shape conformation, one leg of which iswelded to the flattened end 2| of the support [2 and the other end ofwhich has a flag 22 of thin metal to which the filament is welded.

The preferred filament material of my invention comprised a binary alloyof a chromium and nickel which has a specific resistance of 600 ohms ormore per circular mil foot. I also contemplate the use of ternary alloy,however, wherein other elements than nickel and chromium are present,such as cobalt. This filament has a high tensil strength at normalemitting temperature.

A chromium-nickel alloy having 500 ohms per circular mil foot will alsobe satisfactory. So far as I am aware, the more commonly acceptedmaterial for filaments is tungsten or nickel. I have discovered,however, that nickel and chromium provide a material having a highspecific resistance, as well as a low temperature coefiicient ofresistance.

This material may be drawn down to a diameter ofapproximately .0004inch, which seems to be the practical limit of minimum diameter to whichit may be drawn.

It also has a temperature coefiicient of resistance such that it onlychanges in resistance approximately 5 percent from cold to hot, ascompared with nickel or tungsten which varies over 00 percent from coldto hot. Because the resistance is high, it permits a filament of largerdiameter to be used and of shorter length for a given filament currentand voltage. This allows the manufacture of tubes of extremely smallsize having a straight filament, and which tubes are characterized bytheir extreme static stability as compared with prior small tubes. As amatter of fact, prior to this invention, a severe limitationon the useof these tubes was apparent bevoltage source, and the behavior of thefilament provides such consistent results that it becomes particularlyuseful in D. C. amplifier circuits, such as are used in electronicinstrumentation, due to the fact that between tubes a close tolerance ofcharacteristics is attained. This feature is of particular importance inD. C. amplifiers or electrometer circuits where the absolute value ofcurrent is of interest rather than the incremental value such as is usedin A. C. circuits. In such a circuit the control element may have aninput leakage resistance of the order of 10 ohms and a current of 10-amperes and, very often in such circuits, it is desirable to have thefilament become a part of the resistance network. In such cases, if thefilament has a high tem perature coefiicient of resistance, it becomes anon-linear component which makes it impossible to properly balance thecircuit. The filament maintains a relatively constant emission underconditions of varying filament current, thus decreasing the tendency todrift.

It will be noted that the coating of oxides used on my filamentisextremely thin as compared with previous coatings.

The oxides may comprise the usual barium and strontium compounds whichare preferably deposited on the filament by cataphoresis in the form ofbarium and strontium carbonate crystals. I have found that it isdesirable that these crystals provide the best result if they are nottoo small and preferably are of the largest size which will successfullycataphorize and still adhere properly to the filament.

The coating after being is converted to oxides during the activatingstage of manufacture.

applied to the filament invention and in comparison therewith a similarcurve for a socalled hearing aid tube. These curves show the change inemission with cathode heating current which, although often ignored inalternating current amplifier design, becomes of major importance in adirect current instrument. Ihe curve designated VX tube is one made froma tube having the filament of my invention, and the curve hearing aid isthat of a commercial tube available on the market. The hearing aid ispresumed to have a nickel filament. In each instance, the grids andplates of the tubes were connected together to make a diode and thevalues were reduced to common ordinates giving the emission as 100% at200 m. a. plate current, and at rated cathode current to make the curvescomparable. The relative constancy of plate conductance of the VX tubeis apparent. The operating range is wider,

and the change in emission current is considerably less.

It will also be noted that the VX tube cathode produces a nearlyconstant difierence over a wide range either side of rated operatingcathode current. The hearing aid tube does not have I the filament tauta well defined saturation point and the conductance varies greatly withcathode current.

It has been found by graphical analysis that the plate current, in theregion in which the tube is emission limited, varies exponentially; thatis, it follows the expression I =MI;". As a specific instance, the tubeshown in Fig. 3 follows the expression Ip=MIf where M is a constantdepending on the geometry of the tube and power input to the tube.

able to make tubes for the purpose intended which have heretofore beenimpractical of construction. These tubes have a definite and importantuse in connection with the instrumentation of nuclear physics, allowingsimple and rugged instruments to be made and provide consistent and longlife operation.

Having thus described my invention, I claim:

1. A miniature tube for control purposes including an evacuatedenvelope, electrodes disposed in the envelope and including at least ananode and an electron emitting cathode, said cathode being capable ofstable operation with changes in heating potential and comprising asingle short wire of under three-quarters of an inch in length andbetween .0008 and .0004 inch in diameter, means to support said filamentcomprising a lead in wire extending through a press in said envelope anda support extending spaced from the filament, spring means on the end ofthe support connected to said other end of the filament to hold it taut,said filament being formed of an alloy of nickel, chromium and cobalt insuch proportions as to provide a high specific resistance and lowtemperature resistance coefficient and low thermal conductivity.

2. A vacuum tube for control purposes for controlling small currents,including an evacuated envelope, electrodes disposed therein andincluding an electron emitting cathode capable of stable operation withchanges in heater potential con prising an alloy of chromium, nickel andcobalt having a length of between /8 and inch and a diameter of between.0004 and .0008 inch and of high tensile strength, and spring means tohold 3. A sub-miniature tube for control purposes including an evacuatedenvelope, electrodes disposed in the envelope and including an electronemitting cathode capable of stable operation with changes in heatingpotential comprising a single short length of wire formed of an alloy ofnickel, chromium and cobalt in such proportion as to provide highspecific resistance, a low temperature resistance coefficient and havinglow thermal tivit y the use of a filament of my discovery, I am conduc yJOHN A. VICTOREEN.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Number Name Date 1,803,467 Driver et al. May 5,1931 1,812,103 MacRae June 30, 1931 1,839,399 Slepian Jan. 5, 1932

